1. Field of the Invention
This invention relates to a process for reducing formaldehyde exhaust fumes formed during the production of urea/formaldehyde molding resins.
2. Description of the Prior Art
Urea/formaldehyde molding resins are well-known in the art and exhibit excellent properties, including clarity, scratch- and mar-resistance, excellent electrical properties and excellent resistance to moisture, greases, oils and solvents. The molding compositions are easily handled by a molder and have excellent molding characteristics. Urea/formaldehyde molding resins have been used to produce numerous molded articles.
In general, these resins are prepared by a process which includes adding urea to a solution of formaldehyde in amount such that there is a molar excess of formaldehyde and carrying out the reaction at low temperatures (such as in the range of about 75.degree. to 150.degree. F.) and under slightly alkaline conditions (e.g., pH of 7 to 8.5). The reaction pH is usually maintained by the addition of buffer salts or organic amines. The initial reaction results in formation of methylol compounds which are the monomers for a subsequent resin condensation reaction. After several hours of reaction time, the resultant liquid resin is mixed with a filler to provide desired properties in the final cured product. Generally, cellulosic fillers are employed, such as alpha cellulose, which is a fibrous filler that serves to impart strength and moldability as well as reduce internal stresses during the curing of the resins. After the resin and the filler are thoroughly mixed, the resulting impregnated, pulpy, wet mass is dried by spreading the mass onto a continuous perforated conveyor belt which moves through an oven chamber wherein the moisture level is reduced to about 1 to 2 percent. At the end of the dryer, the material is in the form of a hard, caked mass which resembles a thick layer of popcorn and the popcorn sheet is cut and later combined with desired pigments, lubricants and catalysts. The resulting urea/formaldehyde molding compound is an intimately mixed, high bulk, fine powder.
During the drying operation, not only is water evaporated through the drying stacks, but formaldehyde is also found to be a component of the stack gases. The presence of formaldehyde is believed to result primarily from the excess amount employed during the initial mixing with urea to provide the requisite initial molar ratio which is required for the final molding compounds and which varies depending upon the desired properties of the final product.
It would be desirable to reduce the level of formaldehyde fumes discharged from the stacks to minimal amounts. Indeed, environmental agencies require that such fumes be abated to insignificant levels. Conventional approaches to existence of organic materials such as formaldehyde in the stack gases would suggest the utilization of a scrubbing unit. However, scrubbing equipment of such a nature is considerably expensive, and moreover, the reduction of formaldehyde fumes in the stack gases must be accomplished without adversely affecting the properties of the urea/formaldehyde molding compounds.